In the prior art, die casting method is well known as a casting technology to obtain aluminum alloy castings. This die casting method is a casting method to produce castings by filling molten metal in a casting sleeve into a precise metallic die cavity under pressure. According to this die casting method, there are advantages such as highly precise dimensions of castings, beautiful casting surface, availability of mass production and fully automatic production. For this reason, this method has been conventionally used mainly in the production of metal castings which have melting points below that of aluminum alloy.
However, this die casting method has had a problem that the mechanical strength of castings after casting solidification is apt to be deteriorated owing to:
1 Molten metal poured into the casting sleeve is cooled down rapidly within the inner wall of the casting sleeve, generating solidified debris, which is mixed into molten metal and cast;
2 Air in the casting sleeve is mixed into molten metal, causing blister (a phenomenon where mixed and pressurized gas inflates by thermal load to become blistering);
therefore, it cannot be applied to production of strength parts that require high strength.
In order to solve these problems, there are Special Die Casting Methods which include hot sleeve method where casting sleeve is heated in order to prevent the generation of solidified debris in the inner wall of the casting sleeve as described in the above 1, vertical die casting method which prevents air in casting sleeve as described in the above 2 from being mixed into molten metal, and the like. In addition, there is hot chamber die casting method, which is limited to the casting of zinc alloy or magnesium alloy with relatively low melting temperatures. Therefore, this method can not be applied to wide extent.
However, even in the Special Die Casting Methods mentioned above, when speed for filling the molten metal is high, molten metal in the casting sleeve becomes turbulent and catches gas, and is cooled down in the inner wall of the die cavity together with the gas, causing defect and thus deteriorating mechanical and other characteristics. In order to prevent this problem, it is necessary to make the filling speed extremely low, and in this case, insufficient flow of molten metal is caused. In addition, non-solidified portion is extracted during the development of dendrite, causing segregation at the thick wall portions as shown in FIG. 5, thereby deteriorating the mechanical and other characteristics of the cast.
Apart from the various die casting methods mentioned above, Japan Patent Publication No. H3-47951 discloses a die casting method where dies are fixed to form a cavity having a pouring gate at bottom, to which die arranged at the exit of a cylinder is connected so as to form a drawing to limit the flow of molten metal into the cavity. A port to supply molten metal from exterior is arranged at the center of the direction of central axial line of the cylinder equipped with this die, and a punch is slidably engaged, and a casting apparatus is formed. Molten metal is poured into the cylinder from the supply port, and molten metal is kept until liquid phase and solid phase co-exist, then is pushed and pressed by punch through die and into cavity. According to this die casting method, the following effects are expected:
1: The molten melt can be supplied to cylinder at a temperature only just above melting point, which is relatively lower than the temperature in other methods. Therefore, energy can be saved.
2: Since the temperature of molten metal is low, gas absorption is scarce, and there is no need of degassing process, and products have few gas cavity,
3: Molten metal in a state where liquid phase and solid phase coexist is pushed up by punch, so that it is subjected to plastic working in a semi-molten status while passing through the die to form drawing, while the liquid phase and solid phase are mixed. The punch subjects the solid phase to shear force thereby making the casting structure fine. Thus, products with excellent mechanical characteristics can be obtained.
4: Since the molten metal is processed in a semi-molten state, deformation resistance is less compared with forging method, and equipment costs are reduced.
However, in this die casting method disclosed in Japan Patent Publication No.H3-47951, the structure of semi-molten metal is not granulated in the casting sleeve, so that the difference of solute concentration is large, and it is possible that segregation occurs, as shown in variable density in FIG. 6. Even when the molten metal is filled in die cavity, since its structure refinement is insufficient, there is still much to be improved in its mechanical characteristics.
Further, when the speed to fill the molten metal is fast, molten metal in the casting sleeve becomes turbulent and trap gas, and when this molten metal is cooled down rapidly within the inner wall of the die cavity, mechanical and other characteristics are deteriorated, and castings characteristics become uneven. In order to prevent this problem, it is necessary to make the filling speed extremely low. In this case, insufficient flow of molten metal occurs.
On the other hand, with respect to automobiles, the improvement of fuel efficiency has recently become an extremely important problem from laws and regulations in the United States. From this points of view, automobile parts having light weight are needed. Naturally, automobile parts should be sufficiently strong, and from this viewpoint, when making the weight of the parts light by reducing the thickness of the wall, strengthening of raw material becomes an important subject.
However, since there have been problems as described above in the prior die casting method, aluminum alloy castings produced by this die casting method were too insufficient in strength to be applied for production of high strength parts such as automobile parts and the like.